Called the Great Cold Spot, this dark mark is twice as big as Earth, but cooler and more fickle than the planet’s famous (and similarly sized) Great Red Spot. The cool spot sits in Jupiter’s northern regions, not far from the stunning northern aurora in the planet’s atmosphere. The aurora may play a role in creating the newly detected dark mark, researchers report online April 10 in Geophysical Research Letters.

“We can’t be exactly sure how the spot forms,” says study coauthor Tom Stallard, a planetary scientist at the University of Leicester in England. “But we are sure it is there because we observed it numerous times.”

Stallard and colleagues got their first clue that the Great Cold Spot existed when they used the Very Large Telescope in Chile to study an ion of hydrogen in Jupiter’s atmosphere. While mapping the atmosphere’s temperature and density, the team discovered a region about 73 degrees Celsius cooler than the gas around it. Comparing the data with observations from the NASA Infrared Telescope Facility in Hawaii revealed that the dark mark had actually been seen in the same location for more than 15 years.

Though the mark shows up in the same place, its size and shape changes. “Sometimes it is clearly a spot,” Stallard says, “and sometimes it’s not so prominent.” Such shape-shifting may arise because the cool spot is tied to the influx of energetic particles into Jupiter’s atmosphere from the Jovian moon, Io, showers which help generate auroras. On Earth, auroras can produce vortices in the upper atmosphere that are cooler than the surrounding gas. The Great Cold Spot may be a similar kind of swirling weather system that waxes and wanes as the intensity of Jupiter’s northern aurora changes.

“To see weather in this region of the atmosphere is weird,” says Planetary Science Institute researcher Jeffrey Morgenthaler, who is based in Maine. Researchers thought the temperature contrasts caused by the aurora would get smoothed out quickly, he says, so finding such a long-lived vortex in the upper layers of the atmosphere is unexpected.